1. Field of the Invention
The present invention relates to a boundary acoustic wave device using a boundary acoustic wave which propagates along a boundary between a first medium and a second medium that have different acoustic velocities, and more particularly, to a boundary acoustic wave device that includes a piezoelectric substance made of LiNbO3 and a dielectric substance made of a silicon oxide laminated to the piezoelectric substance.
2. Description of the Related Art
Recently, as a band pass filter used in the RF stage of a mobile phone, various surface acoustic wave filters and boundary acoustic wave filters have been proposed. In a boundary acoustic wave filter, a boundary acoustic wave propagating along a boundary between a piezoelectric substance and a dielectric substance is used. Accordingly, since a package having a cavity is not required, the size of the boundary acoustic wave filter can be reduced.
WO2004/070946 discloses one example of this type of boundary acoustic wave device. In the boundary acoustic wave device disclosed in WO2004/070946, electrodes including an IDT are provided at a boundary between a piezoelectric substance and a dielectric substance, and an SH type boundary acoustic wave propagating along the boundary is used.
In the boundary acoustic wave device described above, the thickness of the IDT is set such that the acoustic velocity of the SH type boundary acoustic wave is less than the acoustic velocity of a slow transverse wave propagating through the dielectric substance and the acoustic velocity of a slow transverse wave propagating through the piezoelectric substance. In addition, when LiNbO3 is used as the piezoelectric substance, and when SiO2 is used as the dielectric substance, an electrode thickness range in which the propagation loss is decreased to approximately 0 and a cut angle range of LiNbO3 in which an unnecessary spurious response is decreased are shown.
In a boundary acoustic wave device, depending on the application, a small change in frequency characteristics due to a temperature change, that is, a reduced absolute value of TCF, may be required. In the boundary acoustic wave device disclosed in WO2004/070946, LiNbO3 is used as the piezoelectric substance and SiO2 is used as the dielectric substance. The temperature coefficient of frequency TCF of LiNbO3 is a negative value, and the temperature coefficient of frequency TCF of SiO2 is a positive value. Accordingly, when LiNbO3 and SiO2 are used, the absolute value of the temperature coefficient of frequency TCF can be reduced.
In addition, an electrode thickness at which the propagation loss of an SH boundary acoustic wave is decreased to approximately 0 varies according to the duty ratio of the IDT. For example, when the duty ratio is decreased, in the thickness range of the IDT disclosed in WO2004/070946, the propagation loss is not decreased to approximately 0 and is relatively large in some cases. Thus, even if TCF can be decreased, the loss inevitably increases.
On the other hand, when the duty ratio of the IDT is increased, it is possible to decrease the propagation loss to approximately 0. However, when the duty ratio is increased, the absolute value of TCF increases, and as a result, frequency temperature characteristics are deteriorated. In addition, when the duty ratio is relatively high, the variation in frequency caused by fluctuation in linewidth decreases, and consequently, the yield is advantageously improved.